Microbial content stability and inhibitory effects of antimicrobials on the shelf-life of high moisture dried fruit

Thesis (MSc Food Sc )--Stellenbosch University, 2000.

ENGLISH ABSTRACT: Drying of fruit is an age-old preservation technique that even to this day, remains popular. On the foreign markets there is a trend towards dried fruit products that are softer and more palatable. Based on this, the South African dried fruit industry has launched a range of "soft eating", high moisture dried fruit (HMOF) products. These products have a moisture content of ca 36% (w/w) and a water activity (aw) of ca 0.85, both of which are considerably higher than that of traditional dried fruit. It is this higher moisture content that makes the product susceptible to serious microbial spoilage and colour deterioration in spite of the use of sulphur dioxide. Currently, the shelf-life of these products is approximately three months and to ensure a quality export product, the shelf-life must be at least nine months. During this study the microbial composition of five South African commercially available HMDF products (raisins, prunes, nectarines, cling stone peaches and apricots), was determined. It was found that high moisture dried (HMO) raisins and prunes had the highest bacterial counts, while apricots were more susceptible to yeast and mould spoilage. The most frequently isolated microbes were members of the endospore forming Bacillus group and osmotolerant yeasts like Zygosaccharomyces. Members of the genera Salmonella, Staphylococcus and even anaerobic endospore formers were present in certain samples of the HMO prunes and raisins. From these results it was concluded that the current pasteurisation method of 7 h at 80°C used by the industry is not effective in successfully eliminating spoilage microbes. High moisture dried nectarines and prunes were used in challenging studies, where four indicator organisms (B. cereus, Z. rouxii, Escherichia coli and Penicillium chrysogenum) were selected to evaluate the shelf-stability of these two HMOF products over six months at 5° and 25°C. The results showed higher bacterial counts for the fruit stored at 25°C (1 200 cfu.g ̄ ¹ versus 450 cfu.g ̄ ¹ at 5°C), as well as extensive Maillard browning of the nectarines at the higher storage temperature. Furthermore, it was found that the pasteurisation method (7 h at 80°C) was not effective in eliminating the endospore former B. cereus, P. chrysogenum and Z. rouxii, which may thus lead to serious spoilage implications. The E. coli strain did not survive in either of the HMOF products and no counts were found even before pasteurisation. From these results it was clear that the current pasteurisation method of HMOF products is not effective in terms of the prevention of spoilage, and that storage of the product at lower temperatures would lead to a better quality product. The impact of different preservatives (lysozyme, pimaricin, sorbate, benzoate and an imidazole compound) on the survival and growth of B. cereus and Z. rouxii, were evaluated in nectarine puree prepared from dried nectarines. Lysozyme showed the best inhibitory effect (70% inhibition) on B. cereus over a 12 h incubation period, followed by benzoate, sorbate, the imidazole compound and lastly, pimaricin. The data obtained for the inhibitory effect of the preservatives on Z. rouxii showed that benzoate and sorbate showed. the best growth inhibitory effect (100% inhibition after 24 h). Pimaricin gave a 100% inhibitory effect after 48 h while the imidazole compound gave a 89% inhibition. From the different pasteurisation time/temperature combinations applied to eliminate the endopspore former B. 'cereus, it was found that pasteurisation of 150 min at 90°C showed the best inhibition results. However, the colour of the product was unstable. It was, therefore, decided to also evaluate the use of modified atmosphere packaging (MAP), as part of the product's processing. It was found that the colour of the HMO nectarines was stable and it is thus recommended that a time/temperature combination of 150 min at 90°C together with MAP, is used to ensure a shelf-stable HMOF product.

AFRIKAANSE OPSOMMING: Die droging van vrugte is 'n eeu-oue preserveringstegniek wat vandag steeds baie gewild is. Daar is 'n tendens in die oorsese markte na droëvrugte wat sagter en smaakliker is. Gebaseer op hierdie tendens het die Suid Afrikaanse droëvrugtebedryf 'n nuwe "soft eating", hoë vog droëvrug (HVDV) produkreeks die lig laat sien. Hierdie produkte het 'n voginhoud van ca 36% (m/m) en 'n water aktiwiteit (aw) van ca 0.85. Dit is die hoër voginhoud wat lei tot mikrobiologiese bederf en verbruining. Tans is die rakleeftyd van die produkte ongeveer drie maande, terwyl uitvoer standaarde 'n rakleeftyd van minstens nege maande vereis. Tydens hierdie studie is die mikrobiese samestelling van vyf Suid Afrikaans beskikbare HVDV produkte (rosyne, pruimedante, nektariens, taaipit perskes en appelkose) bepaal. Dit is bevind dat hoë vog droë (HVD) rosyne en pruimedante meestal hoë bakterie tellings het, terwyl appelkose meer vatbaar is vir giste en misiliêre fungi. Die organismes wat meestal geïsoleer is, was lede van die endospoorvormende Bacillus groep en osmotolerante giste soos Zygosaccharomyces. Lede van die genera Salmonella, Staphylococcus en selfs anaerobiese endospoorvormers is gevind in van die HVD pruimedante en rosyne. Vanuit hierdie resultate was dit duidelik dat die huidige pasteurisasie proses van 7 h by 80°C, wat deur die industrie gebruik word, nie effektief is om bederf organismes suksesvol te elimineer nie. Hoë vog droë nektariens an pruimedante is gebruik in uitdaging studies, waar vier indikator organismes (B. cereus, Z. rouxii, Escherichia coli en Penicillium chrysogenum) gekies is, om die rakleeftyd van die twee HVDV produkte oor 'n opbergingstydperk van 6 maande by 5° en 25°C te evalueer. Die resultate het hoër bakteriese tellings vir vrugte wat by 25°C (1 200 kve.g ̄ ¹ versus 450 kve.g ̄ ¹ by 5°C) opgeberg is getoon, asook dat Maillard verbruining by HVD nektariens plaasvind by die hoër opbergingstemperatuur. Verder is bevind dat die pasteurisasie metode (7 h by 80°C) nie effektief was om die endospoorevormer B. cereus, P. chrysogenum en Z. rouxii te elimineer nie, wat ernstige bederf implikasies tot gevolg kan hê. Die E. coli stam het nie in een van die twee HVDV produkte oorleef nie en geen tellings is gevind nie, selfs nie voor pasteurisasie nie. Die resultate het duidelik getoon dat die huidige pasteurisasie proses vir HVDV nie effektief is om mikrobiese bederf te voorkom nie, terwyl 'n lae opbergingstemperatuur aanbeveel word om 'n beter kwaliteit eindproduk te verseker. Die impak van verskillende preserveermiddels (lisosiem, pimarisien, sorbaat, bensoaat en 'n imidasool verbinding) ten opsigte van die groei en oorlewing van B. cereus en Z. rouxii is geëvalueer in nektarien puree. Lisosiem het die beste inhibisie effek (70% inhibisie) t.o.v. B. cereus getoon gedurende die 12 h inkubasie periode, gevolg deur bensoaat, sorbaat, die imidasool verbinding en laastens pimarisien. Die data verkry vir die inhibitoriese effek van die preserveermiddels t.o.v Z. rouxii, het getoon dat bensoaat en sorbaat die beste groei inhibisie getoon het (100% inhibisie na 24 h). Pimarisien het 'n 100% inhibisie getoon na 48 h, terwyl die imidasool verbinding 89% inhibisie getoon het. Van die verskillende pasteurisasie kombinasies wat aangewend is om die endopsoorvormer B. cereus te elimineer, is dit bevind dat 'n pasteurisasie van 150 min by 900G die beste inhibisie resultate getoon het. Die kleur van die nektariens was egter onstabiel. Gevolglik is daar besluit om gemodifiseerde atmosfeer verpakking (MAV) te evalueer as deel van die produk se prosessering. Dit is bevind dat die kleur van die HVD nektariens stabiel was en daar word verder aanbeveel om 'n tyd/temperatuur kombinasie van 150 min by 90°C, saam met MAV te gebruik, om 'n kwaliteit eindproduk te verseker.

Masters